# Paint Calculator: How Many Gallons Do I Need?

> Enter your room dimensions and the calculator subtracts doors and windows, then tells you how many gallons of wall paint, ceiling paint, trim paint and primer to buy.

**Quick answer:** A 12 × 12 ft room with 8-foot ceilings needs about 2 gallons of paint for two coats on the walls (about 333 sq ft after one door and two windows, at 350 sq ft per gallon), plus just under 1 more gallon if you paint the ceiling.

_Last updated: October 4, 2026. Estimates only; confirm quantities with your supplier._

## How to calculate how much paint you need

Paint is sold by the gallon, and one gallon typically covers **300–400 square feet per coat**. To estimate how much to buy, find the wall area, subtract the openings you won't paint, then multiply by the number of coats.

```
wall area = 2 × (length + width) × height
paintable area = wall area − doors (21 sq ft each) − windows (15 sq ft each)
gallons = paintable area × coats ÷ coverage per gallon
```

If you are painting the ceiling, add length × width to the paintable area. Ceilings normally use a separate flat paint, so buy that in its own can.

### Gallons of paint by room size (2 coats, 8 ft ceiling, 1 door, 2 windows)

| Room size | Wall area | Gallons (walls) | Add for ceiling |
| --- | --- | --- | --- |
| 10 × 10 ft | 269 sq ft | 1.5 | +0.6 |
| 10 × 12 ft | 301 sq ft | 1.7 | +0.7 |
| 12 × 12 ft | 333 sq ft | 1.9 | +0.8 |
| 12 × 15 ft | 381 sq ft | 2.2 | +1.0 |
| 14 × 16 ft | 429 sq ft | 2.5 | +1.3 |
| 15 × 20 ft | 509 sq ft | 2.9 | +1.7 |
| 20 × 20 ft | 589 sq ft | 3.4 | +2.3 |

### How many coats?

Two coats of quality paint give an even, durable finish. One coat is only enough when repainting a similar color with a paint-and-primer product. Switching from dark to light, or painting new drywall, usually needs a primer first.

### Choosing a sheen

| Sheen | Best for |
| --- | --- |
| Flat / matte | Ceilings, low-traffic walls (hides flaws) |
| Eggshell / satin | Living rooms, bedrooms, hallways |
| Semi-gloss | Kitchens, bathrooms, trim and doors |

### Tips for buying paint

- Buy all the paint for a room at once and mix cans together ("boxing") so color is consistent.
- Keep leftover paint for touch-ups; label the can with the room and date.
- Rough or textured surfaces absorb up to 20% more paint, so lower the coverage number to about 250–300.
- For exterior work, use the siding area and a lower coverage figure, and see the [siding calculator](../siding-calculator/) for wall area.

### Worked example: three bedrooms

Three bedrooms, each 12 × 12 ft with an 8 ft ceiling, one door and two windows, two coats, with the ceilings painted too and paint covering 350 sq ft per gallon:

- Wall area per room: 2 × (12 + 12) × 8 = 384 sq ft, minus 21 for the door and 30 for the windows = **333 sq ft**.
- Walls: 333 × 3 rooms × 2 coats ÷ 350 = **5.7 gallons**.
- Ceilings: 144 × 3 rooms × 2 coats ÷ 350 = **2.5 gallons**.
- Total about **8.2 gallons**, so buy 9 gallons or two 5-gallon pails.

### What the calculator subtracts

| Opening | Area deducted |
| --- | --- |
| Standard door | 21 sq ft |
| Standard window | 15 sq ft |

These are typical sizes. For large picture windows or double doors, measure and subtract the real area yourself.

### Ceiling and trim

The ceiling is the length times the width of the room. Trim and baseboards use a different paint, usually a harder, glossier finish, and cover about 400 sq ft per gallon in this calculator. Buy ceiling and trim paint separately from wall paint rather than adding them to the wall total.

### Painting mistakes that cost extra

- **Buying exactly one coat's worth.** Most walls need two coats; a dark-to-light change may need primer too.
- **Ignoring surface texture.** Rough or textured surfaces soak up more paint than smooth walls.
- **Mixing batches.** Combine cans of the same color in one bucket (boxing) so slight differences do not show on the wall.
- **Throwing away the leftover.** Keep a small labeled can for touch-ups, noting the room and date.

## Frequently asked questions

### How much paint do I need for a 12×12 room?

A 12×12 room with 8-foot ceilings has about 384 sq ft of wall area before openings. Subtracting one door and two windows and using two coats, you need about 2 gallons for the walls.

### How many square feet does a gallon of paint cover?

A gallon of paint usually covers 300–400 square feet per coat; 350 is a common planning figure.

### How much paint do I need for a ceiling?

A 12×12 ceiling is 144 sq ft; two coats need 288 sq ft of coverage — a little under one gallon.

### Do I subtract windows and doors?

Yes, subtracting them avoids overbuying. This calculator uses 21 sq ft per door and 15 sq ft per window. Skip the deduction if you plan to paint the trim in the same color.

### How many gallons of paint to paint a house interior?

A typical 1,500 sq ft home with about 4,000 sq ft of wall area needs roughly 20–25 gallons for two coats, plus ceilings and trim.

### Do I need primer?

Use primer on new drywall, stains, patched areas and big color changes. Primer coverage is similar to paint — about 300–400 sq ft per gallon.

### How much extra paint should I buy?

Round up to the next whole gallon, which gives a natural touch-up margin. Buy extra if you're painting bare drywall or textured walls.

### Is a quart enough for trim?

A quart (about 100 sq ft per coat) usually covers the baseboards, door frames and window trim of one average room.

## Sources and further reading

- [Lead-Based Paint Renovation, Repair and Painting Program rules](https://www.epa.gov/lead/lead-renovation-repair-and-painting-program-rules) — U.S. Environmental Protection Agency (homes built before 1978)
- [Volatile organic compounds' impact on indoor air quality](https://www.epa.gov/indoor-air-quality-iaq/volatile-organic-compounds-impact-indoor-air-quality) — U.S. Environmental Protection Agency